Radioimmunoassay techniques for arginine vasopressin will be applied to the measurement of vasopressin in humans under a variety of normal and abnormal conditions. Particular emphasis will be placed on investigation of the diurnal rhythm of vasopressin secretion, as related to patterns of sleep, activity, food and fluid ingestion. Examination will be made of the fluctuations of plasma vasopressin levels during the menstrual cycle of normal women. Effects of changes in fluid and electrolyte balance and volume, the effects of various drugs on vasopressin secretion will be examined. Plasma vasopressin levels will be studied in cases of hyponatremia and other disorders of salt and water balance, following surgery, during treatment with positive pressure ventilation, and following treatment with various drugs.